The present invention relates to cross-flow microfiltration water softening for removal of dissolved, suspended and colloidal solids from water to render it suitable for household or industrial use. More specifically, the invention relates to a method and apparatus for lime softening of hard water to remove at least a portion of the hardness constituents, e.g., dissolved salts of calcium and magnesium. As used herein the term "lime softening" refers to water softening processes employing lime, hydrated lime, lime plus soda ash, excess lime, conventional coagulants, and any combination of the above.
Lime softening water treatment processes may date back to antiquity. In the modern era, prior to the 1930's, water softening was carried out as a four step process. First, the hard water was mixed with lime, or lime plus soda ash, in a mixing tank. Second, precipitation and flocculation took place in one or more flocculation chambers. Third, flocculated water then flowed into a settling chamber where precipitated slude was allowed to settle to the bottom and was drawn from the bottom of the settling chamber for disposal. A portion of the sludge so removed was recycled to the mixing tank in order to speed the softening reactions (softening kinetics depend upon seed crystal concentration as well as calcium and carbonate concentrations). Fourth, clarified water overflowed from the top of the settling chamber and was filtered.
One significant advance in lime softening water treatment processes which occurred in the 1930's was the development of what is commonly referred to as the Spaulding contact reactor. This reactor incorporated the above-noted mixing, flocculation, precipitation and settling steps in a single reactor structure. However, separate, conventional through-flow filtration equipment is still generally required in conjunction with the Spaulding reactor. Spaulding reactors are generally large, complex, cumbersome and, hence, quite costly in terms of capital investment.
The through-flow filters conventionally used in conjunction with water softening processes are generally unable to handle high solids loadings. Filtered particles continuously accumulate on and within through-flow filter media. Consequently, the filter flux rate decreases with time (or headloss increases) and frequent backwashing is required to remove the accumulated solids from the filter media. When product water is used for backwashing there is a significant net decrease in total water production. Relatively large volumes of low solids wastewater are also created which must receive some type of further handling. There is also the problem of filter breakthrough.
The present invention constitutes an entirely new and different approach to lime softening. The present invention eliminates the need for the costly Spaulding reactor (flocculator/clarifier) and substitutes, in its place, a simple mixing/recycle tank having an ordinary blow-down valve. Furthermore, in lieu of the separate through-flow filtration equipment conventionally employed in lime softening, the present invention incorporates a cross-flow microfiltration module as an integral part of the lime softening system.
Cross-flow microfiltration is substantially different from through-flow filtration, in that feed water is introduced parallel to the filter surface, and filtration occurs in a direction perpendicular to the direction of the feed flow. Cross-flow microfiltration provides economic benefits that other conventional options do not. Cross-flow microfiltration systems are capable of clarification, filtration, and thickening in one process step. Equipment and installation costs approach those of direct filtration; yet cross-flow microfiltration tion is capable of filtering streams that contain suspended solids concentrations of 10,000 mg/L or higher. Furthermore, cross-flow systems require less space than conventional throughflow systems and provide higher quality filtrate, in terms of suspended material. Other advantages include the following: (1) the ability to turn the system on and off without a lengthy stabilization period; (2) filter breakthrough cannot occur; (3) recarbonation of filtered water is not required; and (4) modular construction yields a large range of flowrate options.
Contrary to the teachings in the art, we have discovered that lime softening may be accomplished in conjunction with tubular, cross-flow microfiltration systems operating at high solids concentration, e.g., as high as 10-12%, by weight, without suffering from the conventionally expected problem of rapid, debilitating scale-up of the microfiltration tubes. The cross-flow microfiltration tubes in the lime softener of the present invention do not, as previously suggested in the art, suffer from rapidly declining flux rates due to scaling (i.e., the build up of an impermeable layer of sludge), and they are perfectly capable of being acid cleaned.
The problems of rapidly declining flux rates, susceptibility to fouling, and resistance to cleaning have been virtually eliminated in a new method of cross-flow microfiltration utilizing thick-walled porous thermoplastic tubes sold under the trademark HYDROPERM.TM.. The filtration characteristics of these tubes combine both the "in-depth" filtration aspects of multi-media filters and the "thin-skinned" aspects of membrane ultrafilters. The porosity of HYDROPERM.TM. tubes results from the open cell reticulated structure of the tube wall. HYDROPERM.TM. tubes differ from conventional membrane ultrafilters, in that they have pore sizes on the order of several microns, wherein the length of a pore is many times that of its diameter. These tubes are described in greater detail, for example, in "HYDROPERM.TM. CROSS FLOW MICROFILTRATION", Daniel L. Comstock, et al., Neptune Microfloc, Inc. Report No. KT 7307, May 1982, and in Report No. 77-ENAS-51 of the American Society of Mechanical Engineers, entitled "Removal of Suspended and Colloidal Solids from Waste Streams by the Use of Cross-Flow Microfiltration", which reports are hereby incorporated herein by reference to the extent necessary for a thorough understanding of the background of the invention.
Feed flow is through the center of HYDROPERM.TM. tubes at a relatively low pressure, typically less than 40 psi. The filtrate is typically collected in a jacket surrounding the exterior tube wall and withdrawn therefrom by a product line. As feed flow circulates through the tube, solid particles are slowly driven with the product flow toward the tube wall. Thus, the concentration of particles in regions close to the wall steadily increases.
In cross-flow filtration systems generally, because the direction of the feed flow is tangential to the filter surface, accumulation of the filtered solids on the filtering medium is reduced by the shearing action of the flow. Cross-flow filtration thus affords the possibility of a quasi-steady state operation with a nearly constant flux when the driving pressure differential is held constant. Unfortunately, this theoretical possibility has not been achieved in practice.
In general, any liquid from which suspended solids removal is desired will contain a wide range of particulate sizes, ranging in effective diameter from several microns down to colloidal dimensions. Because of the "in-depth" filtration characteristics of thick-walled, thermoplastic tubes, such as HYDROPERM.TM. tubes, particles smaller than the largest pore size of the tube may, under certain circumstances, enter the wall matrix. In any event, above a certain solids concentration in the feed, the majority of the suspended solids are retained at the inner wall of the tube and quickly form a dynamic membrane (also referred to as a "filter cake" or "sludge layer"). The dynamic membrane is thought to be largely responsible for the filtration which subsequently occurs.
Those particles initially entering into the tube wall matrix ultimately become entrapped within it, because of the irregular and tortuous nature of the pore structure. As microfiltration proceeds, penetration of additional small particles into the wall matrix is inhibited by the presence of the dynamic membrane. The formation of the dynamic membrane, together with the possible clogging of the pore structure of the tube by entrapped particles, results in a decline in the filtration flux. In conventional systems, this decline is approximately exponentially related to filtration time.
Various cleaning techniques have previously been investigated for restoring the filtration flux value. Such cleaning techniques have involved chemical and/or physical cleaning of the surface of the filter medium. For example, chemical solvents have been used to dissolve the layer-building filtered particles so as to yield a clean, layer-free filter surface. Hydrochloric acid and other acids are examples of solvents commonly being used. On the other hand, a simple physical cleaning technique commonly used is backflushing of the filter medium, i.e., temporary reversal of the filtrate flow direction. This cleaning technique is frequently used in conjunction with cross-flow filtration processes utilizing hollow tubular filters. Another physical cleaning technique employed in the art involves periodically increasing the recycle velocity longitudinally through the porous tubes. (See, e.g., U.S. patent application Ser. No. 319,066.) Higher recycle rates tend to sweep away accumulated deposits, thus minimizing the build-up of the filter cake within the tubes.